Photodo has a few Pentax 645 MTF data (no 645 AF data though) and the best MTF is from the Pentax SMC P-A 645 Macro 120mm/4.0 lens. I compared this to the corresponding Leica S2 lens which has public MTF data too.

D-FA 645 55/2.8 is probably optically the same lens as the A55/2.8 with digital coating/improvements. It was one of the lens that they did not update from A to Fa when the 645N came out. I am not surprised at all with a slightly smaller sensor that 56X42 that they bring out a FA version of this lens now.

No, the D FA and the A versions of the 55mm are not the same optically. The new 55 mm has 9 lens elements, the A has 8 elements.

On another note: The new D FA version is 645 full frame, and can be used on 645 film cameras. Does this hint at a future full frame 645D? Possibly!

Photodo has a few Pentax 645 MTF data (no 645 AF data though) and the best MTF is from the Pentax SMC P-A 645 Macro 120mm/4.0 lens. I compared this to the corresponding Leica S2 lens which has public MTF data too.

This new era of digital MF will bring all of us some very outstanding glass. Seems like the new Leica S2 is more about the glass than anything else.

I would be very cautious with those comparison, all in all, a fair comparison between 2 lenses can be made only on the same body. Here if I'm correct we are comparing film to digital. (Photozone advise not to compare 2 lenses on different systems even though their protocol si quite consistent)

I'm convinced that Leica have done a fine system with their S2, but lets wait for real life tests by people used to the exsiting MF offering to get an opinion on the capabilities of legacy glass.

I would be very cautious with those comparison, all in all, a fair comparison between 2 lenses can be made only on the same body.

I posted the same comparison in my German forum and I was challenged to provide evidence that this comparison be meaningful

So, I made my homework and came up with the following information:

MTF comparisons across systems are indeed valid.

Care must be taken that MTF was measured with capable equipment. Photodo uses a Hasselblad Ealing optical bench which is why results carry across systems. The photozone tests are methodologically inferior. A lens MTF measurement must not involve a camera. How to measure MTF is well defined since hundred years or so.

Some manufacturers quote design specs, not actual measurements (like Canon). I retrieved an interview with the head of optical development of Leica where he explains that the Leica MTF curves are computed. But computed in a way that manufacturing imperfections are taken into account. I checked available data from photodo and leica to verify this claim for lenses which have data from both sources.

Care must be taken that compared curves apply to the same linepair/mm resolution. The finest resolution found in documents varies between 30, 40, 60 and 80 lp/mm. I verified that the photodo and Leica finest curves are 40 lp/mm indeed.

So, you're right, one must be very cautious with those comparisons. And cautious I was.